Field of the Invention
The present invention relates to inspection apparatus and methods usable, for example, to perform metrology in the manufacture of devices by lithographic techniques. The invention further relates to an illumination system for use in such inspection apparatus and to methods of manufacturing devices using lithographic techniques. The invention yet further relates to computer program products for use in implementing such methods.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Prior to applying patterns to a wafer using a lithographic apparatus, the wafer generally needs to be measured and modelled so as to properly align the wafer and to correct wafer deformations during patterning. A critical performance measure of the lithographic process is overlay, the accuracy of alignment of features in two layers in a device (or between features formed by two patterning steps in the same layer). Alignment sensors having multiple color channels are used in the known lithographic apparatus, to try and obtain the best possible position measurements prior to patterning. These position measurements are used to calculate a substrate model for each wafer.
To improve overlay, additional measurements are made of performance on prior substrates that have been patterned, to identify and correct deviations introduced in the patterning step and/or other steps. Various tools for making such measurements are known, including scanning electron microscopes, which are used to measure performance parameters such as overlay.
Recently, various forms of scatterometers have been developed for use in the lithographic field, which allow high volume measurements. Typically these measurements made over many prior substrates, provide far greater spatial detail than can be obtained with alignment measurements made on a current wafer in the course of patterning. Accordingly, types of measurements are used in advanced process control (APC) methods for a modern lithographic production facility. A substrate model based on the measurements of the current wafer provides wafer-specific corrections, while a process model (or multiple models) provides additional corrections to correct for systematic errors in the machine, for example alignment errors. The cause for these errors often lies in other processing steps, like CMP (chemical and mechanical polishing) and etching, which cause a deformation of the alignment marks. This deformation of the mark results in alignment errors that vary from wafer to wafer, when alignment measurements are made in the lithographic apparatus prior to patterning. Because the process model is based on sampling many wafers over time, it can also be provided with (for example) greater spatial resolution and sensitivity to many other variables.
Because the process model is designed to implement variations varying slowly over time, it is not sensitive to wafer-to-wafer variations. The alignment measurements made on each wafer are sensitive to wafer-to-wafer variations. By using a process model in addition to a substrate model, it has been possible to achieve the high overlay performance required for modern device manufacture. Nevertheless, there is a constant quest to improve even further the performance of lithographic processes. This is to improve yield and consistency of existing devices, and to allow even smaller devices to be produced in future.
The inventors have recognized that correlation between the two models can result in over- or under-correction of errors in some cases, so that some overlay error remains, that is in principle correctable.